PHYSIOLOGY. 
335 
the idea of a plaftic nature, which he fuppofed to be ait 
intermediate principle connefting the foul and the body. 
The human body, which is entirely fpongy, is expofed 
through every part to oppofite currents of air and (ire, 
which traverfe and penetrate it, being introduced alter¬ 
nately by the lungs and by the (kin. Hot, cold, denfe, 
rare, and the other fenfible properties of bodies, are only 
the caufes of the phenomena which we perceive, and are, 
as it were, the occafions or accidents that are required to 
keep in play the intelligent force difieminated through 
nature. 
Arijiotle, the difciple of Plato, in his phyfiological 
de&rines, differs but little from his mafter. He attributed 
to the foul three faculties, a nutritive, a fenjitive, and a 
rational, faculty; in the firft of which, life is the only 
principle; in the fecond, feeling is produced; and the 
third is peculiar to man, and is that part of him which 
knows or judges. This part is either an aftive or a paf- 
five intellect, of which the firft may be feparated from the 
body, and is immortal; whereas the fecond perifhes to¬ 
gether with the body. Life, according to this philofo- 
pher,'is a permanence of the foul, retained by the natural 
heat, the principle of which refides in the heart. 
About the period which we are now confidering, phi- 
lofophy was dividedinto two feffs: the Materialifts, who 
attributed the formation of all beings to the fortuitous 
concourfe of atoms ; and the Spiritualifts, who held that 
the foul enjoyed an exiftence anterior to that of the body, 
which was no other than a paffive organ, in which the 
phenomena that previoufly exifled in the foul, in an ab- 
ftra6t latent manner, became evident and fenfible'. Thefe 
two opinions have each fuffered modifications, but ftill 
exift; the fupporters of each not Laving approached one 
ftep nearer towards the fettlement of the queftion than 
the Greek philofophers juft mentioned. 
There is fo much of truth and beauty in fome of the 
fpeculations of thefe philofophers, that one might almoft 
feel inclined to believe that many phyfiological faffs mult 
have been known to them as foundations for their bril¬ 
liant hypothefes. But then, whenever they do mention 
the ftruflure or funftions of particular parts, we find 
them extremely ignorant; and the examples of Hippo¬ 
crates, Herophilus, and Galen, fnovv fo convincingly that 
a clofe cbfervance of faffs rendered at that time little 
afliltance to fpeculation on the nature of life, that we are 
compelled to view them as poetic infpirations, which 
have indeed been fometimes founded on truth. 
Indeed it mull be very obvious, that mere anatomy, 
however accurate; that pathological obfervations, how¬ 
ever numerous; would rather bewilder than affift the 
philofopher in his conjectures, till the phyfical and meta- 
phyfical fciences were confiderabiy advanced. It w ; as 
not till the fciences of chemiftry, of mechanics, and of 
metaphyfics, had each in their turn been erroneoufly 
applied to explain the nature of life, that the exafl 
degree of their feparate operations became evident; and 
it was not till the difunited multitude of clinical obfer¬ 
vations began, by the aids of anatomy and phyfics, to be 
connected into general propofitions, that the fpeculations 
of the Greek philofophers were lupported or denied with 
advantage to the progrefs of fcience. Thus in the reve¬ 
ries of Paracelfus and Van Helmont we fee the impofiibi- 
lity of explaining by chemical doffrines the aff ions of the 
animal economy; and, while we admire the wifdora of 
the difcoverer of the circulation, and of the ufechanic 
philofophers of the 15th century,'yet we fee in their 
deduction from the faCts they had promulgated, made by 
Boerhaave, that anatomy and mechanics inform us only 
of the coarfer movements of our wonderful frame. 
It was in the 18th century that Hoffman and Stahl firft 
pointed out the necefity of looking to other powers 
oefides thofe treated of by chemifts and mechanifts for 
the explanation of the laws of life. 
Hoffman faw, that in the living body we ought not to 
feparate the principle of vitality from the general proper¬ 
ties of matter. He believed that the principle, fufcep- 
tible in itfelf of activity and motion, was fufficient for all 
the occafions and all the functions of the body which it 
animated. The animal body was not, in his eyes, an 
hydraulical elaftic machine, formed of folids and canals, 
differing only in fize, form, elafticity, and force. He faw 
that, if the folids act upon the fluids, thefe muft, in their 
turn, re-aCl upon the folids; and that life could fubfift 
only by thefe mutual aCtions and re-aCtions. The effen- 
tial caufe of life, according to Hoffman, is the progreflive 
motion of the blood, occafioned by the impulfe of the 
heart, and kept up by the alternate contractions and 
dilatations of the veflels. Thefe contractions and dila¬ 
tations are the confequence of the force of an elafticity 
inherent in the vafcular fibres, and this force is ftill far¬ 
ther promoted by'the different ftruC'tures of thefe elaftic 
fibres, which is fuch, that they can be penetrated by the 
blood and the nervous fluid. This laft fluid he imagined 
to be compofed of aerial and etherial particles enveloped 
in a certain portion of a very pure fubtile lymph, that 
ferved them as a vehicle. By this fluid the cavities of 
the nerves are filled; and it conliitutes the fenfitive foul, 
in which refides the feat of the paflions. Now, all the 
functions, even thofe which we attribute to a fentient 
principle, are the effeCt of phyfical powers, whofe mecha- 
nifin lias, however, fomething more fublime and more 
exalted with refpeCt to the animal operations than to 
others. If all the nervous, vafcular, and membranous, 
parts, preferve a moderate degree of aCtion, and a mode¬ 
rate ftate of tenfion and relaxation, the folids are fubjeCted 
to ofcillatory motions which balance each other, and 
produce a proper equilibrium in the fyftem. In this 
ftate, all the operations of the body and the mind take 
place with proper regularity ; and this happy harmony, 
by affuring to the animal the entire plenitude of its ex¬ 
iftence, becomes the foundation of health. This degree 
of moderate tenfion is always more or lefs altered in a 
ftate of difeafe. 
The grand principle of Stalil was, that, after admitting 
other forces befides thofe engaged in mechanics, he con- 
fidered that an intelligent foul direCts the operation of 
each of thefe forces, as we (hall prefently fee. In determi¬ 
ning the limits between medicine and the other phyfical 
fciences, this author commences with feparating from the 
former all thofe principles which, though true in them- 
felves, have no relation to the nature of that fcience, 
which he conliders as originating in obfervation alone. 
The knowledge of the phyfical ftate of the animal body 
cannot, he thinks, throw any light, eitheron the injuries 
to which it is expofed, or on the means of preventing or 
removing them. Confequently it is of little ufe in medi¬ 
cine, and has no right to govern an art theobjeff of which 
is to remedy thofe injuries that threaten the human bod}-. 
He proves that living bodies are freed from the neceffarv 
laws of mechanics, becaufe all their actions tend to one 
common end; an end which embraces the whole chain 
of the movements eflential to life, and the means elta- 
blifhed for its prefervation. The human body, by means 
of this mixture of mechanical and vital powers, tends 
naturally to felf-deftrudtion ; but, on the other hand, the 
organic ftrufture to which is attached the exercife of the 
actions peculiar to the human fpecies, is founded on this 
mixture. It is therefore necefiary that the body fliould be 
in a ftate of refilling this tendency, in order that it may 
be luftained ; and, as the corruptibility inherent in its 
nature purfues it through every period of its exiftence, 
the oppofing aflion, necefiary to prevent the corruption 
from taking place, mult alfo be exercifed without 
intermiflion. It is this preferring' afliou that cor.ftitutes 
the effence of life. 
The prefervation of the body is indeed effefled by a 
fort of mechanical aftion; it requires the corporeal or¬ 
gans as its instruments, and it depends on different co- 
3 exiftent 
